A conventional out-of-plane Micro-Electro-Mechanical Systems (MEMS) device, such as an accelerometer, may include one or more silicon proof masses mechanically coupled to and suspended from a substrate, typically glass, using one or more silicon flexures. A number of recesses etched into the substrate allow selective portions of the MEMS device to move freely within an interior portion of the MEMS device. In certain designs, substrates can be provided above and below the silicon structure to sandwich the proof masses between the two substrates. An out-of-plane MEMS device may be used to determine an out-of-plane linear acceleration by measuring exerted acceleration forces which displace portions of the proof mass.
The movable portion of the proof mass of an accelerometer, referred to herein as a rotor, and an adjacent, fixed portion of the MEMS device, referred to herein as a stator, are separated by a gap. The gap is typically filled with air or another suitable gas such that the proof mass rotor is free to move in response to acceleration of the MEMS device. The gap between the proof mass rotor and the fixed stator define a capacitance.
In an open loop operated MEMS device, changes in the capacitance, induced by movement of the proof mass rotor with respect to the stator, is detectable. Based upon the detected change in capacitance, the amount of linear acceleration of the MEMS device can be determined.
However, the movement of the proof mass rotor necessarily causes a geometry change between the rotor and the stator. Since capacitance between the rotor and the stator is proportional to the coupled rotor/stator area divided by the separation distance squared [Force=fn(Area/Distance2)], changes in the geometry between the rotor and the stator affect force in a non-linear manner. Such non-linear changes in force significantly complicate the process of determining the acceleration of the MEMS device and may introduce error in the determination of the acceleration. Accordingly, it is desirable to substantially prevent and/or control movement of the rotor with respect to the stator so that the nonlinear changes in force between the rotor and the stator are minimized.